Metal-to-glass seal



Dec. 26, 1933.

c. v. LITTON 1,940,870

METAL-TO-GLASS SEAL Filed Sept. 15, 1930 I i 5 Fl 5 l 4 a 2 l i P q ,V, 1/4 I z EXPOSED 5URF'ACE5 OF METAL NICKEL PLATED /3 I 1 I I 11 1 I L i .T

5 I F: GE 4 INVENTOR CHARLES LITTON TORNEY Patented Dec. 26, 1933 a r a a s I Nl'TE-DSTATES omen v ME'rAL-To-GLAss SEAL,

Charles V. Litton, Redwood. City, -Calif., assignorto Federal Telegraph Company, San ;Fran. cisco, Calif. a corporation of California j'Ap'plication September 15,1930

'Se'rial -No.' 481,932

- sclaimsl (01. 250-275) This invention relates to metal-rto-glass seals flap-joined or lap-weld anodeseal is disclosed and the'art of making thesameQ It has to'do United States'patent to. Housekeeper No. more especially, though not exclusively, with 1,560,690. l 7 metal-to-glass seals in'devices such as high power vSeals ofthe. lap-joined type as disclosed in vacuum tubesand particularly seals'oflargeor theprior art appear to be totally impracticable relatively large diameter. I as anode seals, although'qui'te 'practicablefor The present inventionis-fthe outgrowth of a other purposes. If it were not forinexorable limparticularly difiicult probleminherent in the proita'tions' as to the kinds of glass and metals caduction of. high power'vacuum tubes, wherein a p'able'of being "used, a Iapri'oined seal of the 10 part of the enclosing envelope is made, of glass ordinary type could probably be made toserve the ,20 The seals specifically referred 25 nominated hereinafter;

' whilst the remaining'part isof'metaland constif purpose very satisfactorily." But unfortunately tutes the anode element. Structures of this cha'rthe tube designer cannot select his materials pure-, acter. must of necessity be provided with'a vacuum 1y on the basis of their coefiicients of expansion tight seal joining'the metal'and glass portions'of and other jfactors'vital only tolthe seal. iself.

16 the envelope; and it is jwith'seals'. for that purpose The glass and metal selected must be capable or that the "present invention is mainly-concerned-- formingastrong weld, and the glass must have although it will be evident that the'js'eals to be certain properties essential to its use in'avacuum hereinafter described are not limited in their utiltube. Likewise the metal selectedmust have suit.- ity to that particular purpose, ablepropertiesinadditiontobeingffwettable? by I to above are'comthe glass. f 76 monly known as anode sealsibecaufse they serve Ifsufiiciently thin metal'were employed, an anto connect the 'glassportion of the envelope with ode seal of the ,lapejoined type, could doubtless that part thereof, which-constitutes the anode be made,-I which, purely as .a seal,",would 'nieet, all elementofithe tube. Such seals may be thus-derequirements.v That is to "say, it would probably I notrupture under the forces of contraction and so The principal 'di'ificulty, "th anodei 'seals'has expzinsion-butit would notbe acceptable because been breakage caused by the iorcesof expansion of'its deficiencies' 'in other respects.

.. and contraction. That is to say, difierences of The present in ention ovecomes the, .deexpansion and contraction between the'inetal and ficiencies referred .to inthatit permitsthe mak- .30 glass, accentuated by large and rapid tempering of a .1ap j0ihed a a w th imtal .of

ature variations, tend't'o engender-stresses which substantial thickness. This is accomplished will either break the glass. or rupture th'el weld. through the use of what will be termed a reinforc- One way of avoiding. anode seal breakage is to ing element, cr,-more specifically,,'a-reinforcing make the metal part of such extreme thinness ring, the function of which'is to counteractin I 35 and ductility that it will yield to those forces withpart the tendency of the metal to expand. and

out rupturing the weld or breaking the glass. contract, thereby, relieving the main weldof the Phat expedientovercomes the one difficulty but seal, and likewisthe glass, element of thejseal, gives rise .to another, namely; that the mechanof the excessive stresses which would otherwise ical strength of the tubeis seriously impaired; A result in, breakage, ,From this point on, the in- 40 large power tube has considerable weight'and ,Ventioncan be'bestdescribed by'refe'rence to'lthe must withstand'shipment and sometimes rough accompanying drawing whereinz' handling. And what is more,'such tubes are quite Figj l is a longitudin l sectional view through expensive. Hence, the importance of providing an anode'seal wherein 'theglass element of the an anode seal of ample mechanical stre'ngthwill seal is welded to the exterior surface or a metal 5 be apparent, That, together with theprov'ision tubethe latter formingthe'metal element of the loo of a seal that can be conveni'entlyfand,economiseal. I a M V I cally produced, constitutes the principal object of Fig.2 is av siniilar'showingof an alternative seal the present invention. design wherein the glass element is welded to the .Thekind of seals with which ,i'nvehtibn'is ,interior'surfaceof the metal tube;

50 "concerned are of the tubular'Flap-joined or .lap-' I In each of. the two figures, 1 indicates the glass weld typ'e-those terms'beingsynonymous and element, tubularin form, which is sealed, that is, intended to distinguish irom'seals of the sci-called weldedto a metal tubular element 2 Element 2, a fork type and other types which aremore'or together with that part oflelement l Iwhich'is wless obviously not inthe samecategory An exjoinedthereto', maybe considered'as formin'gthe 5 ample from the prior artfof whatishere term'eda seal'.per se; Inea'ch instance there is "shown ,a

reinforcing element in the form of a ring 3. In the structure of Fig. 1 the ring 3 is inside the metal tube 2, while in Fig. 2 it is disposed outside.

The metal tube 2 should be quite thinthat is, as thin as practical considerations permit. But as previously indicated it does not need; to be so thin that its mechanical strength is unduly impaired. The thickness of the metal is of .the order of a few thousandths of an inch. Tubes ranging in thickness from seven thousandths to fifty thousandths of an inch have been used. e

In each instance, the reinforcing ring is welded to the metal tubethis operation preferably being performed concurrently with the welding to the metal of the-glass element 1. Y r

The reinforcing ring 3 is, in general, made of glass having suitable properties, but there is no reason to suppose that other materials than glass would not serve the purpose, as for example, porcelain. Hencein referring to glass, it is intended th'at equivalent materials, such as porcelain, be included. The same applies to the tubularglasselement 1, which obviously might be madeof other materials such as porcelain.

It hasb'ee'n stated that the reinforcing ring is welded to thejmetjal tube, but since the weld in thatcase servesonly as an anchoring means, it becomes evident that there maybe' other suitable expedients, such as'mechanic'al anchoring; That does not hold true, however; of the joint between elements 1 and 2. That joint must be absolutely a r ti h The element 3 hajs been shown and referred to as a ring, but'th'a't obyiou'sly must be regarded as a generic term. \The reinforcing element may take numerous 'formsysuch as that ,of a disc or a spider, and it is not necessarily continuous at pe i 1e v-j.; Q

The reinforcing ringS'is, in each instance, disposed in contraposition tothe weld between eleinents land that termpis meant that the reinforcing ring is directly opposite the weld referredto, instead of be ng'atsome position sublycwas t the composition of the elements 1, 2

and 3, no exact specificationscan be given which would be applicable to allc'ases. Injmaking a choice of materials, one must frequently be. guided by',factorsf havin'g no direct bearing on the seal itself. For jfe'xample,' one may belimited as to choice of glasses due'to the temperatureswh ich h'ave to betaken into .accounta'nd because it may have to meet other 'requirementspeculiar to the particular case. Likewise, it is necessary to chooseametal having desirable characteristics in all necessary respects as'well as beingwettable.

Sim'f r cdnsiderations of course apply to the rei i s r l .Hav'in'g' determined what materials may be usable, it is desirableto selectglass andmet'alfor elements 1 and 2 respectively having as nearly as possibleequ'al coeflicients of expansion, thus reducing to a minimum the forces of expansion and contraction likely to be' developed. Thematerial outjo'f which'thereinforcing ring 3 is made should be selected with a view to obtaining expansion and contraction therein such as to offset the tendency of the metal element to expand and contract excessively. ,ThusQif the glass of element 1, Fig. 1, has a lower coefficient of expansion than that of the metalof element 2, the coefficient of expansion of the reinforcing ringshould be less than thatof'themetal. But it does not follow as a matterof course thatthe reinforcing ring should be made of the same kind of glass as that of the .,.When the reinforcing ring is inside as in Fig. 1,

where the temperature may rise more rapidly or reach a higher point than that of the element 1,

itis likely to be desirable to use a glass having a lower coefficient of expansion. The converse might be true with respect to the seal shown in Fig; 2.

From what has been said, it is manifest there is much occasion for the exercise of good judgmentand engineering ability in the designing of a seal of this kind, and furthermore that it may be unavoidably necessary to experiment before an entirely suitable design to meet the needs of any particular case can bearrivedat;

For vacuum tube anode seals such as those to which particular reference has been made,fpure copper isrecommended for the metal element 2. The purity of the copper is quitevital, and likewise its freedom from defects such as blowholes. Since the metal is necessarily quite thin, blow holes may be disastrous. Likewise, the presence of oxides in the copper may be severely detrimental. Copper of suitable purity and freedom from blow holes may be produced by first melting in an atmosphere of hydrogenforre: ducing the oxides and thereafter melting under vacuum for removing occluded gases. In practice, it has been found necessary to resort to these expedients in order to avoid an excessive number of failures. A'defective seal may not be detectable until the vacuum tube is completed and put in operation. It is much more economical ,to take all possible precautions against failure than to have them develop after the tube is completed. 1 f 7 From the information already given those skilled in the art should have no more difficulty L than should normally be expected in producing a satisfactory seal in accordance withthe present invention, but forthe sake of completeness of 'disclosure some specific examples of successful anode seals will be given. One such seal approximately in accordance with Fig. 1 was two inches in diameter at the weld. The element 2 was of copper tubing made as before described and .009 thick. The flange of element 1 was its thick and wide. The reinforcing ring 3 was j of like dimensions. Elements 1 and 3 were made of the same kind of glass having a coefiicientof expansion of .0362 x 10- per degree centigrade.

Other successful seals have been made with .coppertubing as thick as .040". ,In these cases, the outside glass had a coefiicientjof expansion of .0362x 10* per degree centigrade, while the inside glass had a coefficient of expansion of .034 x 10 per degree centigrade. The latter seals were also of 2'. diameter.

As a measure of. precaution. against oxidation occurring during the welding operation, it is very desirable to nickel'plate those surfaces of element 2 which are not to be covered by the glass, and, in addition, a small margin in each instance that is to be covered by the glass. The glass will not adhere as firmlyto the nickel as it does to the copper, but it makes'an unsatisfactory weld its metal. Thus it is obviously preferable 'to have lee a small part of the metal surfaces to which the welds are to be made nickel plated.

It isnot essential that the element 1 be made entirely of one kind of glass. The flange portion thereof may be made of one kind of glass and the remainder of another kind, if so desired. It will a be noted that the flange part of element 1 taken by itself is a glass tube. This may be and is preferably attached to the element 2 indepenpart of the present invention, since the 'inven tion resides in the seal, per se, which is not primarily concerned as to the nature of the device in which it may be incorporated.

I claim:

1. A vacuum tube seal comprising a copper cylindricalshell, an insulating cylindrical shell member welded to one surface of'said' copper shell, and a second insulating cylindrical shell member welded to the other surface of 'said' copper shell, said members being independent of each other, out of contact with the ends of said copper shell and co-axial therewith.

2. A vacuum tube seal comprising a copper cylindrical shell, an insulating cylindrical shell member welded to one surface of said copper shell over a portion removed from its edges, and a second insulating cylindrical shell member welded to the other surface of said copper shell and also removed from its edges, said members having a coefiicient of expansion difiering from that of the copper shell and conjointly restricting the expansion of said shell. I

3. A seal in accordance with claim 6 characterized by the fact that said members contact said copper shell over unequal surface areasone of which is completely super-imposed upon the other.

CHARLES V. LITTON. 

